Quote from Thezzaruz >>

Quote from AntonSirius >>

Incorrect, in the case of things like True Strike, Booming Blade etc. You are explicitly doing both

Quote from AntonSirius >>

War Magic allows you to replace any attack made during the Attack action with a 1 Action Wizard cantrip, with no other restrictions

I think that what @Plaguescarred is getting at is that your problem is one of timing.

Because even if one accepts that a weapon attack made during a cantrip can be the "attack of the Light property" then the Light property still only gives you an attack as a Bonus Action. And the Nick property doesn't move that attack to the Attack Action until the time when you make the attack. 

 That can't be true, because you don't start taking a bonus action, and then it magically becomes part of your attack action.  The attack action would be over and done with by that point.  It has to be the potential to make the Light/Nick attack that moves it into the Attack Action, not beginning a totally separate action type.

And that solves the timing problem, because since it must be in the attack action before you started taking the attack, its an attack you can replace.

Light

When you take the Attack action on your turn and attack with a Light weapon, you can make one extra attack as a Bonus Action later on the same turn. That extra attack must be made with a different Light weapon, and you don’t add your ability modifier to the extra attack’s damage unless that modifier is negative. For example, you can attack with a Shortsword in one hand and a Dagger in the other using the Attack action and a Bonus Action, but you don’t add your Strength or Dexterity modifier to the damage roll of the Bonus Action unless that modifier is negative.

Nick

When you make the extra attack of the Light property, you can make it as part of the Attack action instead of as a Bonus Action. You can make this extra attack only once per turn.

War Magic

When you take the Attack action on your turn, you can replace one of the attacks with a casting of one of your Wizard cantrips that has a casting time of an action.

Exceptions Supersede General Rules

General rules govern each part of the game. For example, the combat rules tell you that melee attacks use Strength and ranged attacks use Dexterity. That’s a general rule, and a general rule is in effect as long as something in the game doesn’t explicitly say otherwise.

The game also includes elements—class features, feats, weapon properties, spells, magic items, monster abilities, and the like—that sometimes contradict a general rule. When an exception and a general rule disagree, the exception wins. For example, if a feature says you can make melee attacks using your Charisma, you can do so, even though that statement disagrees with the general rule.

So, at this point, when the Eldritch Knight takes the Attack action at levels 7-10 (at 11+, they get more normal attacks), we have:

1. One attack with no restrictions other than attack 1 or 2 (or both) must be with a Light weapon.
2. One attack with no restrictions other than attack 1 or 2 (or both) must be with a Light weapon.
3. One attack with a different light weapon later in the turn as a Bonus Action as part of the same action. You can only make this attack once per turn.

Now, here is where we have the first point of contention. As far as I know, there is still dispute as to whether you have to use the Nick weapon for the attack during the attack action (attack 1 or 2) or the attack from the Light property (attack 3). I believe the consensus is that either attack can be with a Nick weapon and it is fine because the Nick property does not specify one way or the other.

If your interpretation is that the Nick weapon must be used as the attack from the Light property, do not proceed further. You cannot replace it with War Magic, because when you replace the attack with a Cantrip, you have no longer made an attack with a Nick weapon. You need to have the attack available in order to replace it with a spell.

If your interpretation is that the Nick weapon can be one of the standard attacks from the Attack action, then as long as attack 1 or 2 is with a Light, Nick weapon, attack 3 is available. In this interpretation, Nick makes the weapon attack from the Light property part of the Attack action. Nick is an exception and supersedes the Light property and the Attack Action. War Magic is an exception that supersedes the Attack Action, the more general Nick property, and, because of Nick, the Light property.

In this case, the Attack action from the above Eldritch Knight would look like this:

1. One attack with no restrictions other than attack 1 or 2 (or both) must be with a Light, Nick weapon.
2. One attack with no restrictions other than attack 1 or 2 (or both) must be with a Light, Nick weapon.
3. One attack with a different light weapon later in the turn as a Bonus Action as part of the same action. You can only make this attack once per turn. One of the Eldritch Knight's Cantrips.

In this case, actually attacking with a Nick, Light weapon (that you have Mastery for currently, of course) secures the attack from the Light property as part of the Attack action and it is therefore eligible to be replaced.

• Is this RAW? I think so. Again, if you think the Nick weapon must be used as the attack from Light property, this doesn't work.
• Is this RAI? I don't know.
• Does the Light property's restriction on preventing the Attribute Modifier to damage unless it is negative still apply? Maybe. War Magic doesn't do anything that explicitly overrides that aspect so if the Cantrip involves an attack (spell attack or weapon attack), technically the restriction still applies. If you cast a Cantrip that uses a Save, it wouldn't be affected.

As always, if you are counting on this tactic, discuss it with your DM beforehand.

Horde Breaker is not pertinent here because it's not the feature in question (Nick vs War Magic), and because it has no reference to the Attack action whatsoever to be replaceable by War Magic anyway.